1. Field of the Invention
The present invention relates to a communication apparatus for transmitting a signal subjected to orthogonal frequency division multiplex (hereafter referred to as OFDM) modulation and receiving a signal subjected to OFDM modulation.
2. Description of the Related Art
An OFDM modulation has been put to practical use as one of modulation methods for transmitting digital data of a comparatively large amount by radio or the like. For example, in a comparatively narrow region, such as in a home or in an office, as shown in FIG. 1, a video signal (digital video data) outputted by a video signal source 1 including a tuner for receiving television broadcasting and a reproducing device for reproducing a video program recorded on a recording medium is supplied to a radio transmitting device 2. The video signal is subjected to modulation processing in the radio transmitting device 2. A resultant OFDM modulated signal is subjected to radio transmission in a predetermined frequency band from an antenna 3. The radio-transmitted signal is subjected to receiving processing in a radio receiving device 5 connected to an antenna 4. The OFDM wave of the received frequency band is subjected to demodulation processing to yield a video signal. The received video signal is supplied to a video recording and reproducing device 6, and recorded on a predetermined recording medium such as video tape. Or the received video signal is supplied to a television receiver 7, and subjected to picture receiving processing. In this case, it is also possible to reproduce the video signal recorded by the video recording and reproducing device 6, and supply the reproduced signal to the television receiver 7.
In such a system configuration, radio transmission between the antenna 3 connected to the radio transmitting device 2 and the antenna 4 connected to the radio receiving device 5 is conducted by using a signal subjected to OFDM modulation. As a result, radio transmission of digital data of a large amount can be conducted favorably and efficiently. The OFDM modulation method is a method for transmitting transmission data as a multi-carrier distributed among a plurality of sub-carriers. Radio transmission of data of a large amount can be conducted efficiently by this modulator method.
An example of a configuration for conducting OFDM modulation for transmission in the radio transmitting device 2 is shown in FIG. 2. A transmission signal (digital data) obtained at an input terminal 2a is supplied to a serial-parallel converter 2b, and converted to parallel data every predetermined unit. The parallel data obtained from the serial-parallel converter 2b is supplied to a memory 2c for interleave. By changing the order of writing to the memory 2c and the order of reading from the memory 2c, interleave processing for changing the data arrangement is conducted. The parallel data subjected to the interleave processing is supplied to an inverse Fourier transform circuit (IFFT circuit) 2d. By using computation processing using an inverse fast Fourier transform, orthogonal transform processing for transforming the time axis to the frequency axis is conducted. The parallel data subjected to the orthogonal transform is supplied to a parallel-serial converter 2e. Resultant serial data is supplied to an output terminal 2f. The data obtained at the output terminal 2f is frequency-converted to a predetermined transmission frequency band by a transmitting processing system, and subjected to radio transmission.
An example of a configuration for receiving a signal thus radio-transmitted and demodulating the signal in the radio receiving device 5 is shown in FIG. 3. A signal of a predetermined frequency band is received, and frequency-converted to an intermediate frequency signal or the like. The intermediate frequency signal is obtained at an input terminal 5a. The data obtained at the input terminal 5a is converted to parallel data at every predetermined unit by a serial-parallel converter 5b. The converted output is supplied to a Fourier transform circuit (FFT circuit) 5c. By using computation processing using fast Fourier transform, orthogonal transform processing for transforming a frequency axis to a time axis is conducted in the Fourier transform circuit 5c. The parallel data subjected to the orthogonal transform is supplied to a memory 5d for de-interleave. By changing the order of writing to the memory 5d and the order of reading from the memory 5d, de-interleave processing for changing the data arrangement and restoring the original data arrangement is conducted. The parallel data subjected to the de-interleave processing is converted to serial data by a parallel-serial converter 5e, and the serial data is supplied to an output terminal 5f. 
Thus, each of the modulation configuration at the time of transmission of the signal and the demodulation configuration at the time of reception of the OFDM signal is a circuit configuration of a comparatively large scale. The communication apparatus for transmitting and receiving the OFDM signal has a problem that the circuit scale becomes large. In other words, each of the inverse Fourier transform circuit for generating an OFDM modulated signal by computation processing of inverse Fourier transform at the time of transmission and the Fourier transform circuit for demodulating the OFDM modulated signal by computation processing of Fourier transform at the time of reception needs a circuit having a configuration of a large scale, resulting in a large-sized circuit scale. Furthermore, in the case where interleave is conducted prior to transmission as shown in FIGS. 2 and 3, memories for interleave processing and de-interleave processing are also needed. Thus, the radio communication apparatus for transmitting and receiving the OFDM signal has a problem that the circuit scale becomes very large.
An object of the present invention is to make it possible to conduct the processing for generating the OFDM modulated signal at the time of transmission and the processing for demodulating the OFDM modulated signal at the time of reception by using a simple configuration.
In order to solve the problems, in accordance with the present invention, inverse Fourier transform processing for transmission and Fourier transform processing for reception are conducted in one Fourier transform circuit, and when reading out data which is set in an output register of the Fourier transform means and forming serial data, the data is read out at the time of transmission in an order different from an order in which the data is read out at the time of reception.
In accordance with the present invention, processing for modulating a signal to produce an OFDM modulated signal at the time of transmission and processing for demodulating the OFDM modulated signal at the time of reception are conducted by a set of Fourier transform circuits and other circuits connected before and behind the Fourier transform circuits.